The present invention relates to a new group-III nitride tunneling junction structure.
Generally, a tunneling phenomenon in highly doped P-N junction diodes has attracted many interests. Particularly, a tunneling phenomenon in GaAs-based devices which can be easily doped with a high concentration of P or N is frequently used in the fabrication of devices which have low resistance and low power consumption by using electric currents caused by electrons as a substitute for currents caused by holes with low mobility, because of good resistance in a reverse bias state. This has been discussed in “Buried tunnel contact junction AlGaAs-GaAs-InGaAs quantum well heterostructure lasers with oxide-defined lateral current”, J. J. Wierer, etc, Appl. Phys. Lett. 71(16), pp. 2286–2288, October, 1997.
Also, InGaAs with low band gap can be interposed between p-n junctions, so that a tunneling potential barrier can be lowered, thus increasing tunneling probability. This phenomenon has been discussed in “High current density carbon-doped strained-layer GaAs(P+)-InGaAs(n+)-GaAs(n+) p-n tunnel diodes.”, T. A. Richard, etc, Appl. Phys. Lett, 63(26), pp. 3616–3618, December, 1993.
In GaN-based nitride semiconductor devices (LED, LD, HBT, FET, HEMT, etc), the formation of low-resistance p-ohmic contacts necessary for improving the performance of the devices encounters many difficulties because of the low conductivity and large band gap of magnesium-doped p-GaN. In an attempt to overcome this problem, studies have been performed in order to reduce power consumption by inserting a reversely biased GaN p-n tunneling junction into a GaN-based LED. See U.S. Pat. No. 6,526,082, “P-contact for GaN-based semiconductors utilizing a reverse-biased tunnel junction”. Also, there was an attempt to reduce a loss caused by a semi-transparent conductive film in LED, by using highly n-dopable GaN itself as the conductive film. See “Lateral current spreading in GaN-based LED utilizing tunnel contact junctions”, Seong-Ran Jeon, etc., Appl. Phys. Lett. (78), 21, 3265–3267, May, 2001.
In order to realize an effective tunnel junction, high concentration doping must be possible first of all. In GaN, there were many efforts to increase the doping level of p-type GaN (e.g., p-type InGaN which is doped at a very high concentration, superlattice structure, and 3D grown GaN), but devices having the tunnel junction in GaN undergo a given tunneling barrier, thus causing an increase in operation voltage [Chih-Hsin Ko, etc, “P-dwon InGaN/GaN Multiple Quantum Wells Light emitting diode structure grown by metal-organic vapor phase epitaxy”, Jpn. J. Appl. Phys. 41(2002) pp. 2489–2492]. However, since GaAs- or InP-based group III-V compound semiconductors can be easily doped with a high concentration of P, highly p-doped GaAs or graded p-type AlGaAs may be grown on a low concentration of P-GaN so as to lower the potential barrier, thus reducing resistance. See U.S. Pat. No. 6,410,944 issued to Song Jong In, “Epitaxial structure for low ohmic contact resistance in p-type GaN-based semiconductor.”
Generally, in GaN-based optoelectronic devices, it is difficult to make an electrode with low contact resistance due to the large band gap and low conductivity of p-type GaN. On the other hand, in the case of n-type GaN, high concentration doping is possible and an electrode with good resistance characteristics can be easily formed thereon by plasma treatment, etc., without an annealing process. Thus, high power efficiency, high operation speed and high reliability can be ensured by electric currents caused by electrons with a higher mobility than that of holes, which flow by means of an electrode formed using the tunneling phenomenon of a P-N junction, other than hole currents flowing by means of an electrode formed directly on p-type GaN. A generally known GaN-based tunnel junction structure is shown in FIG. 1. In this case, in order to increase tunneling currents, a high concentration of an electron layer 12 (n>1019/cm3) and a high concentration of a hole layer 13 (p>1019/cm3) are required between the n-Al(x)Ga(y)In(z)N layer 14 and the p-Al(x)Ga(y)In(z)N layer 11. As the size of an electric field formed in a depletion layer produced at the junction between the two semiconductor layers increases, the tunneling currents increase. In addition, if a strained InGaN layer is added to the junction, a piezo-electric field will be formed at the junction and will have a positive function to increase the tunneling current. This is disclosed in U.S. Pat. No. 6,526,082, “P-contact for GaN-based semiconductors utilizing a reverse-biased tunnel junction.”
In order to realize an effective tunnel junction, high concentration doping must be possible first of all. In GaN, there were many efforts to increase the doping level of P-GaN (e.g., p-type InGaN which is doped at a very high concentration, superlattice structure, and 3D grown GaN), but devices having the tunnel junction in GaN undergo a given tunneling barrier, thus causing an increase in operation voltage. See “P-dwon InGaN/GaN Multiple Quantum Wells Light emitting diode structure grown by metal-organic vapor phase epitaxy”, Chih-Hsin Ko, et al, Jpn. J. Appl. Phys. 41(2002) pp. 2489–2492.